The disclosure relates to a laser driver, and more particularly to a laser driver with automatic laser power control of laser diodes.
Laser diodes have been widely adopted in optical communication due to their high brightness and low dispersion.
Laser diodes are generally driven by a laser driver. A laser driver receives a digital signal to control or vary the current flowing through the laser diode, thus controlling the brightness thereof (i.e. optical output power) and a light emitting signal is then generated. A receiver identifies the digital signal according to the received optical output power. For example, when the optical output power of the laser diode is P1, which is relatively large, it represents a logical high state ‘1’. When the optical output power of the laser diode is P0, which is relatively small, it represents a logical low state ‘0’.
The greater the difference between P1 and P0, the better the identification of transmitted data. The quality of the light emitting signal is examined based upon the extinction ratio (ER=P1/P0). That is, the greater the ER, the better the quality of light emitting signals.
Laser diodes, however, have drawbacks, such as aging effect and temperature effect. That is, when a laser diode is driven with a constant current, the optical output power generated thereby decreases with the increase of operating temperature. Consequently, there is feedback mechanism in the related art to detect the light emitting signal of the laser diode for varying the driving current thereof to fix the average optical output power of the light emitting signal.
Though such feedback mechanism fixes the average optical output power, it, however, fails to solve the impact on ER which is incurred by aging and temperature effects. FIG. 1 shows characteristic curves of the optical power to driving current in a laser diode. With the increase of the operating temperature or aging of components, the optical power-to-driving current characteristic curve may vary from a curve with a greater slope, L1, to a curve with a smaller slope, L2. Assuming the feedback mechanism of the related art fixes the average optical output power of the laser diode, Pavg, the average driving current hence varies from I1avg to I2avg automatically with the variation of the characteristic curve from L1 to L2. The difference between different driving currents (i.e. the magnitude of modulation current), however, does not vary in response to the increase of temperature or aging of components, that is, I11−I10=I21−I20. With reference to FIG. 1, the optical output power of light emitting signal varies from P11 and P10 to P21 and P20 respectively. It can be easily found that with the variation of slopes, ER2 (=P21/P20) is smaller than ER1 (=P11/P10). In other words, if only the average driving current is controlled, the quality of the light emitting signal degrades in response to the increase of temperature or aging of components.